When radiant energy from the sun strikes a solar collector, some of the energy is reflected or transmitted and lost, and the remainder is either absorbed or re-radiated into the atmosphere. Most of the sun's energy is emitted at wavelengths below 2.0 microns and a substantial amount of such energy is normally reradiated into the atmosphere at a longer wavelength, such as infrared radiation. It is desirable, therefore, for solar collectors to have a high capacity for energy absorption below 2 microns in combination with low emissivity at the longer wavelengths.
Blackbodies are known to absorb a significant amount of energy in the solar spectrum but, unfortunately, also re-radiate most of the energy in the infrared (IR) spectrum and, therefore, are generally unsatisfactory as collectors.
Preferred absorptive coatings or films, which optimize solar absorptance and inhibit emissivity in the longer wavelengths, are generally known as "selective absorbers". A good selective absorber may have, for instance, a solar absorptance level (.alpha.) approaching a value of 1.0 and a thermal emittance or emissivity (.xi.) approaching zero.
The principal factors affecting absorptance, emittance and thermostability are (1) the physical and chemical properties of the absorber film, (2) the nature of the substrate to which they are applied, and (3) the nature and functional properties of the diffusion barrier or interlayer between said film and said substrate.
Metals such as silver, gold and aluminum, for instance, have low emissivities, but are low absorbers of solar energy and, alone, are not generally useful for collector purposes. They can, however, be incorporated into a selective solar absorber collector as a low emissivity component. Some metals, however, are unstable at higher operating tempertures, resulting in rapid deterioration of their low emissivity properties, and substantially diminishing the effectiveness of the collector. Silver films of earlier solar energy collectors, in particular, exhibit a tendency to agglomerate at higher operating temperatures.